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Vitamin K Absorption in HorsesBy Kentucky Equine Research Staff · April 26, 2016

Vitamin K not only plays an important role in blood-clotting but also in a number of other physiological processes such as bone metabolism, immune function, programmed cell death, energy production, and the development of spermatozoa. In fact, 14 different proteins dependent on vitamin K to function properly have been identified so far. Considering there are three different forms of vitamin K, where do horses get the most vitamin K?

During a recent Australasian Equine Science Symposium, vitamin K absorption was a well-reviewed topic. Two separate studies were presented, each highlighting basic information about this fat-soluble vitamin.*,**

  • Vitamin K1, also called phylloquinone, is found in green plants, and only small amounts are present in cereal grains;
  • Vitamin K2, menaquinones, are a family of different forms of vitamin K that are synthesized by intestinal bacteria; and
  • Vitamin K3, the synthetic form of vitamin K added to equine feeds.

According to Nutrient Requirements of Horses, last published by the National Research Council in 2007, vitamin K deficiency in horses due to inadequate consumption has not been described. The requirements of dietary vitamin K are unknown, but horses are assumed to fulfill their requirements through consumption of forage, such as pasture and hay, and intestinal bacterial production.

Recognizing that there is more to vitamin K than just its blood-clotting properties, Wayne Bryden, Ph.D., professor of animal science at the University of Queensland School of Agriculture and Food Science, and colleagues conducted absorption studies to better understand the uptake of this important vitamin in horses.

Horses were administered different forms of oral vitamin K. Blood samples were subsequently obtained at various time points following administration and vitamin K levels (including K1, MK-4 as a representative form of K2, and K3). They found:

  • The highest K1 concentrations in the blood were obtained following administration of a water-soluble vitamin K product containing both K1 and K2;
  • The bioavailability of K1 was only 0.14% following administration of K1 orally, but the water-soluble formulation resulted in a bioavailability of 0.45%;
  • K3 was well absorbed following oral administration; and
  • Minimal absorption of orally administered K1 occurred from the large intestine.

“These studies indicate that horses appear to have K1 as the predominant circulating form of vitamin K and that horses have poor uptake of vitamin K from the large intestine, suggesting that bacterial-produced vitamin K is not an important source of the vitamin,” summarized Kathleen Crandell, Ph.D., an equine nutritionist for Kentucky Equine Research (KER).

In addition, these studies show that vitamin K metabolism in horses is different than in other mammals, like rats and humans, in which K1 is converted to K3 and MK-4.

*Skinner, J.E., A.J. Cawdell-Smith, J.R. Biffin, et al. 2014. Vitamin K absorption in the horse: Intestinal uptake of different vitaminers. In: Proc. Australasian Equine Science Symposium, Vol. 5. Gold Coast, Queensland, Australia. p. 31.

**Regtop, H.K., A.M. Talbot, J.R. Biffin, et al. 2014. Vitamin K absorption in the horse: Does absorption occur from the hindgut? In: Proc. Australasian Equine Science Symposium, Vol. 5. Gold Coast, Queensland, Australia. p. 32.